Shut-off nozzles have long been employed in injection molding machines to prevent "drool" and the problems associated therewith, to shorten the cycle time, and for a number of other advantages. The present invention relates to certain improvements in shut-off nozzles for injection molding machines such as those described in prior U.S. Pat. No. 4,082,226, and sold by EMI Plastics Equipment Corp. of Jackson Center, Ohio, under the trademark PERFORMER.TM.. These shut-offs utilize an axially movable pin which shuts off right at the nozzle tip.
The movable shut-off pin is reciprocated by a fairly large pancake piston-cylinder assembly as shown in the patent, or by a pair of smaller pneumatic cylinder assemblies arranged diametrically of the pin. In either case the axially reciprocating pin is moved within a nozzle body by actuator arms which pass through a diametral slot in the nozzle body. Passages for the plastic melt or shot are provided around and parallel to the pin. These passages extend from diametrically enlarged chambers at each end of the nozzle body where the male-female threaded connection is provided to the nozzle tip or tip extrusion, or the machine adaptor. Such shut-offs perform effectively for most standard injection molding machines which operate at below 20,000 psi. However, some newer machines operate at pressures ranging from 30,000 to about 43,000 psi or more. At such pressures, with the usual threaded connection, the high internal pressure creates leak problems, particularly at the diametrically enlarged chambers which provide enlarged surface areas upon which the increased pressure acts.
It has also been discovered that abrupt changes in the flow path cross section tend to create a problem which is termed resin burn. Resin burn creates carbon which tends to flow into the part being molded which in turn could create a faulty part. Such carbon may have a tendency to build up plugging or partially plugging smaller openings such as the nozzle tip orifice or many parts of the mold runner system.
Accordingly, to provide an effective shut-off nozzle at such extremely high pressures it is important to avoid a conventional male-female threaded connection such as a pipe thread between the machine adaptor and the nozzle body, and also between the nozzle body and nozzle tip or tip extension. It is also important to avoid large surface areas in the resin flow path, particularly between the machine adaptor and the nozzle body. It is also important to avoid abrupt changes in flow path cross section, particularly from the machine adaptor to the nozzle body.